1. Field of the Invention
The present invention relates to an automatic focusing apparatus which can be applied to an auto-level, a theodolite or a transit instrument etc., having a telephotographic system.
2. Description of the Related Art
A surveying instrument, such as an auto-level, a theodolite or a transit instrument etc., is provided with a collimating telescope, a level, and scales for measuring a rotative angle and/or an elevational angle. The surveying instrument is first horizontally set and its horizontal and vertical angles are then adjusted, so that an operator may observe a collimation point or object through a collimating telescope.
A typical surveying instrument having a collimating telescope is provided, in order from the object side, with an objective lens, a focusing lens and an eyepiece. The position of the focusing lens is adjusted according to the distance from the object, so that an image of the object may be formed on a focusing plate (reticle). The operator may thus observe, via the eyepiece the image superimposed on the reticle.
For example, provided that the range of the object distance is between 1 m--infinity (a very wide range in comparison with a telescope or binoculars), and that the focusing lens is a concave lens, the approximate amount of movement of the focusing lens is 30 mm. The focusing lens is normally driven through the operation of a rotative knob. If the amount of movement of the object image, i.e., the amount of movement of the focusing lens, is set smaller in regard to the rotational angle of the rotational knob, the amount of movement of the object image becomes smaller in comparison with the rotational angle, but it takes some time to move the lens. On the other hand, if the amount of movement of the object image is set larger with respect to the rotational angle of the rotative knob, the amount of movement of the object image becomes too big as compared with the rotational angle, which leads to difficulty in stopping the object image on the reticle.
Additionally, if the object is distant, the object image moves a large amount forward and backward through a slight operation of the rotational knob in the rotational direction. On the other hand, if the object is positioned relatively close to the focusing lens, the amount of movement of the object image becomes smaller as compared with the amount of rotational of the rotative knob, thus the rotational knob must be moved a great deal for the purpose of moving the object image on the reticle. In such a case, it is difficult to determine whether the object aimed at is in a front focus or a rear focus position, which may lead the operator to rotate the rotational knob in the wrong direction with respect to the focal point. In any event, a conventional auto-level suffers from the defect that it takes too much time to focus.
In such circumstances as described above, a collimating telescope is provided with a so-called passive automatic focusing apparatus. As is well known, this (passive) phase difference type of automatic focusing apparatus is provided with an AF line sensor that receives light by a pair of CCD line sensors. The pair of CCD line sensors are positioned at an optically equivalent position to a focusing plate. An optical path of the telephotographic system diverges, and the luminous flux of the object in the diverged optical path is split into two beams. The split beams respectively form object images on re-forming lenses, and the re-formed object images are received by the above-mentioned CCD line sensors. The defocus amount is detected by the image data output from the pair of CCD line sensors, based on the phase difference (interval) of object images formed on the pair of CCD line sensors, then the focusing adjustment is performed.
However, a conventional AF sensor may suffer in the case that the design optical distance or the focusing position of the lens is not obtained in practice, due to errors in assembling, or to humidity or temperature. Consequently, the phase of an object formed on the CCD line sensor is displaced from the original phase, which results in an error, (whether plus or minus), in the defocus amount. Consequently, the focusing may not be performed on the object aimed at due to focusing based on the erroneous defocus amount.